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Scientific Papers 



OP THB 



Bureau of Standards 

S. W. STRATTON. Director 



No. 377 

INTERCRYSTALLINE BRITTLENESS 

OF LEAD 

BT 

HENRY S. RAWDON, Physidrt 

Bureau of Standards 



APRIL 6, 1920 




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Scientific Papers 



OF THE 



Bureau of Standards 

S. W. STRATTON, Director 



No. 377 

INTERCRYSTALLINE BRITTLENESS 

OF LEAD 



BY 



HENRY S. RAWDON, Physicist 
Bureau of Standards 



APRIL 6, 1920 




PRICE, 5 CENTS 

Sold only by the Superintendent of Documents, Government Printing Office 
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V 



i\ 



Bo" •f i>« 

MAir 28 1920 



^0-2(oin 



INTERCRYSTALLINE BRITTLENESS OF LEAD 



By Henry S. Rawdon 



CONTENTS 

Page 

I. Introduction 215 

II. Corroded lead sheathing 216 

III. The allotropism of lead 219 

IV. Experimental embritttement of lead 220 

1 . Immersion in solutions of lead salts 220 

2. Klectrolysis of lead in concentrated nitric acid 228 

V. Explanation of results 231 

VI. Summary 232 



I. INTRODUCTION 

The relation between the course or the path of the fracture of 
metals and alloys produced in service or as a result of certain 
laboratory tests and the crystalline units of which such materials 
are composed is of utmost importance. The fractmre of normal 
material is, in general, intracrystalHne ; that is, it consists of a 
break across the grains rather than of a separation between them. 
An intercrystalline fracture indicates either that the metal is of 
very inferior quality or that the break occurred under very 
unusual conditions; for example, at a very high temperature. 
The usual mechanical tests, when applied to metals of the type 
which breaks with an intercrystalline fracture, merely measure 
the coherence of adjacent grains for one another and reveal little 
as to the real properties of the metal itself. 

Even such a soft, plastic substance as lead, under suitable con- 
ditions, may be rendered so weak and brittle that it can be easily 
crumbled into powder by the fingers, although the constituent 
grains have lost none of the intrinsic properties of lead. Various 
erroneous explanations of this behavior of lead have appeared in 
the scientific literature, the change being described usually as 
an allotropic one. The importance, in an industrial sense, of a 
proper explanation of this type of the corrosion of lead justifies 
the description of the type of metal deterioration which follows. 

215 



2l6 



Scientific Papers of the Bureau of Standards 
II. CORRODED LEAD SHEATHING 



[Vol. i6 



In Figs. I and 2 are shown two typical specimens of corroded 
lead sheathing selected from materials submitted to the Bm-eau 
for examination. Fig. i shows a section of the sheathing of an 
aerial cable; Fig. 2, that of the lead covering of a subterranean 
feeder line of an electric-light system. In this case the deterio- 



^ 




(n) Unetched, surface view, X 3 




(6) Same material, etched with concentrated nitric acid. X 3 

Fig. I. — I niercrysialline brittleiiess in commercial lead 

The material was part of the covering of an aerial cable; the details 
of service were not reported, other than that it occurred in a hot 
climate. The break may have been the result of the combined 
effect of stress and corrosion 

ration had proceeded to a much greater extent than in the case 
shown in Fig. i , so that certain portions of the metal could easily 
be crumbled into a coarse, gray powder by the fingers. This 
was true, in particular, of those portions of the sheathing where 
the surface discoloration showed it to have been immersed in 
water. The surface appearance of the embrittled lead is shown 
in Fig. 3. The sheathing in such spots was so weak and brittle 
that it could be easily crumbled into a gray powder, although the 



Rawdon] 



Brittleness of Lead 



217 



appearance of the original surface was but little changed. In 
Fig. 4a some of the particles of the embrittled lead are shown, 
many of which are of a definite crystalline form. Each particle, 




Fig. 2. — Corroded lead-cable sheathing 

A portion of the embrittled cable sheathing was flattened out. The 
photograph shows the exterior of the sheath ; the surface has been 
broken up by a network of "alligator cracks." The metal can be 
easily crumbled with the fingers. One-half natural size 

when tested on a glass plate with a small pestle, was found very 
malleable, and when cut the characteristic color of lead was 




Fig. 3. — Corroded lead-cable sheathing 

The small granular spot in Fig. 2 is here shown at a higher magnifica- 
tion. The rough crystalline appearance of the metal is very evident. 



revealed. That the lead was still malleable was illustrated by 
rolling portions of the embrittled lead into thin strips. By 
suitable care the metal was rolled as thin as 0.008 inch; however, 



2l8 



Scientific Papers of the Bureau of Standards 



[Vol. 16 



the metal tore badly during the rolling, as shown in Fig. 46. 
Evidently the coherence of crystals for each other comprising the 
lead sheathing had been greatly weakened, but the crystals them- 
selves still retained the characteristic properties of lead. 

In Fig. 5 is shown the appearance of a cross section of the 
sheathing. This was taken from a portion in which the metal 
was only partially embrittled. That the method of the corrosive 




(a) 




(6) 
Fig. 4. — Corroded lead-cable sheathing 

(o) The corroded metal can be easily crumbled into small grains as 
shown. Many of the grains show definite crystalline faces. X 8 

(6) The grains composing embrittled lead are still malleable. Pieces 
can be rolled into thin sheets if care is taken. The strips tear badly 
in rolling, however. Above strip was reduced from 0.12 to 0.008 
inch in thickness. X i 

attack is intercrystalline in its natxu"e, as well as the fact that the 
action begins on the outer or exposed svirface of the sheathing 
and proceeds inwardly, is very evident from the micrograph shown 
in Fig. 5. 

A chemical analysis of the material showed that there was no 
essential difference between the corroded and the uncorroded 
portions and that the lead contained a considerable amoimt of 
tin. A determination of the melting point of the material con- 



Rawdon] 



Brittleness of Lead 



219 



firmed the indications of the chemical analysis that the embrittled 
portions of the lead are essentially the same as the unattacked 
parts. These data are summarized in Table i below: 

TABLE 1. — Composition and Melting Point of Embrittled Lead 



Tin 

Lead 

Melting point 



Embrittled 
portion 



Per cent 

1.09 

98.3 

325. 4° C 



Unattacked 
portion 



Per cent 
1.05 
98.23 
325. 6° C 



The melting point of pure lead is 327.4° C (621.3° F). 




Fig. 5. — Corroded lead sheathing 

Cross section through a portion of material of Fig. 2. which was only partially- 
embrittled. The action begins at the outside and the intercrystalline metal 
is first attacked. Specimen is unetched. X 30 

III. ALLOTROPISM OF LEAD 

An allotropic form of lead similar in its properties to the well- 
known "gray tin" has been described by Heller i. Creighton ' 
has described a method entirely different from that used by Heller, 
by which this "gray lead" appeared to be produced. In brief, 
Heller's method consisted in the immersion of bright sheets of 
lead in solutions of lead acetate which contained appreciable 
amounts of nitric acid. The transition was stated to have begun 
at the end of two days and to have been complete in about three 



1 Hans Heller, Zeit. f. Phys. Chem., 89, p. 761; 1913- 

* H. J. M. Creighton, Jour. Am. Chem. Soc, 37, p. 2064; 1915- 



220 Scientific Papers of the Bureau of Standards [V6i.i6 

weeks. The lead lost its original strength and ductility and dis- 
integrated into particles, gray to gray black in color, which could 
be pressed easily between the fingers into a pulverulent mass. 
The change was stated to occur to a very sHght extent in lead 
immersed in solutions of pure lead acetate. The addition of a small 
amount of nitric acid, however, increased the rate of change very 
markedly. I^ead solutions other than that of the acetate were 
found to permit the change to occur. However, the presence of 
a small amount of nitric acid appeared to be necessary in every 
case. The inoculation of pure sheet lead with some of the gray 
form did not lead to any detectable change in the inoculated 
specimen. The lead used throughout the experiments was de- 
scribed as not containing " other metals in amounts worth mention- 
ing. Neither silver nor tin was present, and only a trace of iron 
was found." 

The method described by Creighton consisted in the electrol- 
ysis of lead in nitric acid (sp gr, i .42) , the lead being the cathode. 
The cathode was described as having increased slightly in volume 
and having lost its former malleability and firmness. The lower 
portion was found to have been completely changed. Small par- 
ticles which could easily be detached could be rubbed into a fine 
powder or pressed together into a soft mass. Cohen and Helder- 
man ^ have noted changes in the density of clean lead filings 
immersed for about three weeks in a lead-acetate solution, and 
have interpreted the density changes as evidence of an allotropic 
change occurring within the lead. An immersion of three weeks 
caused an increase in density from 1 1 .322 to 1 1 .342. Heating the 
lead after immersion has the effect of slightly lowering the density. 
These changes are, however, very much less in magnitude than 
those described by both Heller and Creighton. 

IV. EXPERIMENTAL EMBRITTLEMENT OF LEAD 

The method described by Heller and Creighton; by which the 
allotropic forms of lead may be produced, was tested out for the 
purpose of comparing the granular lead which may result from 
corrosion during service with the allotropic form. 

1. IMMERSION IN SOLUTIONS OF LEAD SALTS 

In the description of the method given by Heller lead-acetate 
solutions were used for most of the immersions, as was also done 
by Cohen and Helderman. In the preliminary trials a solution 

^ E. Cohen and W. D. Helderman, Verslag K. Akad, Wetenschappen, 23, pp. 754-761; 1914. 



Rawdon] 



Brittleness of Lead 



221 



similar to that described by Heller was used: Water, looo cm'*; 
lead acetate, 400 g; nitric acid (sp gr, 1.16), 100 cm''. However, 
in most of the experiments normal solutions of neutral lead ace- 
tate were used, and the concentration of nitric acid -was varied in 
different tests from 0.5 N to 2 N. 

Two types of lead were used, a commercial lead of ordinary 
grade and a pure lead of exceptionally high purity. The compo- 
sition of the two grades is given in Table 2. 

TABLE 2.— Composition of Lead Used ^ 



Constituents 



Commer- 
cial lead 



Antimony 

Iron 

Tin 

Copper 

Nickel 



Per cent 

0.07 

.02 

.14 

(<^) 

(0 



High- 
grade 
lead 



Percent* 

0.003 

.004 



Constituents 



Zinc 

Bismuth. 
Silver... - 
Lead 



I^ommer- 
cial lead 



Per cent 



99.72 



High- 
grade 
lead 



Per cent 6 
(.<:) . 
<« 99.993 



a The author is indebted to J. A. Scherrer, of this Bu.-eau, for these analyses, as well as for the succeeding 
one (p. 229) 
'' The analysis was carried out upon a sample of loo g. 
c Not detected, 
d By difference. 

In the series of tests described below thin sheets of pure lead 2 
by 4 cm by 1.5 mm were immersed in the different solutions for 
24 days. In order to relieve any internal stresses set up by the 
cold rolling, which might influence the behavior of the lead when 
immersed in the electrolyte, the lead was annealed before use for 
approximately three hours at 200° C after being rolled into sheets. 

The following solutions were used, and the specimens were 
suspended vertically by means of silk thread, so as to be exposed 
to the solution on all sides : 

(i) N lead acetate, 2 N nitric acid. 

(2) A^ lead acetate, j.6 N nitric acid. 

(3) N lead acetate, 0.8 N nitric acid. 

(4) A^ lead acetate, 0.5 AT nitric acid. 

(5) N lead acetate, no nitric acid. 

(6) A^ lead acetate, 1.6 A^ nitric acid. 

The commercial-lead sheet was immersed in solution No. 6 
and pure lead in each of the others. A volume of 50 cm ^ of solu- 
tion was used for each specimen. In all cases in which nitric 
acid was used a sHght evolution of gas occurred, particularly in 
the first part of the test period. This was identified as nitric 

160847°— 20— 2 



22 2 Scientific Papers of the Bureau of Standards [Voi.jg 

oxide. The evolution of the gas from the pure lead immersed in 
solution No. 2 (A^ lead acetate and 1.6 N nitric acid) was at the 
approximate rate of 20 cm ^ in 24 hours. When the commercial 
lead was immersed, the evolution was considerably faster. With 
solutions containing less nitric acid the evolution of gas was pro- 
gressively less as the concentration of the acid was decreased. It 
could hardly be detected in the 0.5 A'' nitric-acid solution, and 
none whatever was observed in the solution of lead acetate to 
which no acid had been added. 

A sHght deposit or "sludge" formed in the bottom of the flask 
as the action of the solution upon the lead proceeded. In the 
cases where pure lead was used the amount was very small and 
increased in amount as the concentration of acid increased. The 
deterioration was most rapid in the case of the commercial lead, 
and a relatively large amount of sludge formed. Pure lead 
immersed in solution No. 2 {N lead acetate, 1.6 N nitric acid) 
gave 0.02 g deposit after 5 days' (24 hours each) immersion, 
while commercial lead in the same solution produced 0.52 g in 
the same time. The sludge produced from the commercial lead 
contained some rather large-sized particles, similar to those shown 
in Fig. 4a, which were malleable and could be flattened out on a 
sheet of glass. (See Figs. 76 and yc.) The solution continued to 
act upon these particles, as was indicated by the evolution of the 
gas, and finally only a gray flocculent powder remained. The 
sludge formed from the pure lead was of a flocculent nature and 
gray in color. This is evidently an oxidation product. It is well 
known that bright lead soon changes its color in water solution 
by oxidation, particularly if a trace of acid be present.* 

The solutions in which the lead specimens were immersed grad- 
ually turned from colorless to a lemon color, the intensity of the 
color being proportional to the concentration of the acid initially 
present. At the end of the 24-day period, during which the speci- 
mens were under observation, it was a simple matter to place the 
solutions in correct order with respect to the initial acid con- 
centration by the color of the liquid; no change of color could be 
detected in the simple lead-acetate solution. 

The appearance of the specimens at the end of the 24 days' 
immersion is shown in Fig. 6. The specimens which were in the 
solutions of the greatest acid concentration were very much 
roughened and embrittled; those in the solutions of less acid 

* W. Vaubel, Zeit. angew. Chemie, 25, p. 2300. 



Rawdon] 



Brittleness of Lead 



223 



concentration merely showed evidence of etching on the surface 
by which the crystalHne structure was revealed. In some cases 
(Nos. 3 and 4) the action had been somewhat greater at the sharp 
corners and edges, so that a rough, crystalline appearance was 
produced at such points. 





vm 








Fig. 6. — Lead immersed in nitric acid solution of lead acciaiefor 2^ days 

Specimen i.pure lead immersed in a solution of lead acetate {N), nitric acid (aiV) 
Specimen ;, pure lead immersed in a solution of lead acetate (TV), nitric acid (,i.6N) 
Specimen 3 , pure lead immersed in a solution of lead acetate (iV) , nitric acid (o.SN) 
Specimen 4, pure lead immersed in a solution of lead acetate (N), nitric acid (o.sAO 
Specimen 5, pure lead immersed in a solution of lead acetate (,V) 
Specimen 6, commercial lead immersed in a solution of lead acetate {N), nitric acid 

(1.6 iV) 
All are slightly larger than natural size 

With specimen No. 4 (immersed in 0.5 A'^ acid), a very faint 
etch pattern on only one side of the sheet was observed, the 
remainder still showing the marks due to rolling. The specimen 
immersed in the solution of lead acetate containing no acid showed 
no evidence of etching; the surface, here as in the others, was 
covered with a slight, gray "bloom" which could be easily wiped 
off. Slight traces of "lead trees" at the sharp comers of the 
specimens were noted in one case of immersion in lead acetate. 



224 



Scientific Papers of the Bureau cf Standards 



[Vol. i6 




(a) 







T« 






iSf .)* i'.. 



\ 




(^) 




Fig. 7- — Embrittled commercial-lead sheathing 

The strip cf commercial lead (Table 2) was immersed in a solution of 1000 cm'' 

water, 400 g. lead acetate and 100 cm' concentrated nitric acid four days 

(96 hours) : 
(a) Surface view of specimen. X 8 
(fc) Crystals which became detached and formed sludge on bottom of flask. 

X8 
(c) Crystals similar to those of (6) that have been flattened. The crystals still 

show characteristic properties of lead. X & 



Rawdon] 



Brittleness of Lead 



225 




S- ■j-^'iw ^xfc^i. ■•■^^ 




■^ •> ,>^-- ' 







(a) 



(5) 





(c) (d) 

Fig. 8. — Intercrystalline brittleness induced in pure lead by immersion in nitric acid, 

solution of lead acetate 

(a) Specimen 2, Fig. 6; (6) specimen 3, Fig. 6; (c) specimen 4, Fig. 6; id) specimen 5, Fig. 6. All have 
been bent at an angle cf approsdmately 150° to reveal intercrystalline weakness 



226 



Scientific Papers of the Bureau of Standards 



[Vol. i6 



The behavior of the different sheets when bent sharply indicates 
clearly that a more profound change occurred in the material 
than was indicated by the appearance of the surface. This is 
shown in Fig. 8. In each case, including those specimens the siu-- 
face appearance of which appeared to be suggestive of no appre- 
ciable change, the metal cracked and revealed a series of inter- 




(a) 




(6) 
Fig. 9. — Pure lead showing intercrystalline embrittlement 

A sheet of pure lead (Table 2) was immersed in a solution of lead acetate (.V) and 

nitric acid (o.siV) for 10 days 
(a) Surface view of the specimen after rubbingto remove the slight "bloom" or 

deposit; tmetched. X 15 
(6) Same specimen after bending through 150°. The fissures which are revealed are 

truly intercrystalline. X 15 

crystalline breaks, thus indicating that an intercrystalline brittle- 
ness of the lead had resulted from the action of the electrolyte. 
These intercrystalline breaks are best revealed in specimens im- 
mersed in lead-acetate solutions containing little or no acid. In 
the solutions of higher acid concentration the attack of the metal 
along the crystalline boundaries is great enough, so that the speci- 



Rawdon] 



Brittleness of Lead 



227 



men is merely roughened. The preferential attack of lead 
along the crystalline boundaries during immersion in a so- 
lution of lead acetate (N) and nitric acid (0.5 A'') for 10 days 
is shown in Fig. 9. The surface has the appearance of being 
very slightly etched; upon bending, however, wide fissures 
formed between the crystals parallel to the direction of bending. 




(a) 










'- , , '■r ,^ 



iV^ 






(6) 
Fig. lo.^Pure lead showing intercrystalline emhrittlement 

A. sheet of pure lead (Table 2) was immersed in a solution of lead acetate (^V) and 

nitric acid (i.6iV) for 12 days 
(a) Specimen after a few minutes' immersion. X 8 
(6) Same specimen after 12 days' immersion in the solution. The area shown is not 

the same as that of (a). Conspicuous fissures have formed around most of the 

crystals. X is 

The more rapid attack along the crystal boundaries of metal 
immersed in solutions of high acid concentration is shown in 
Fig. 10. The lead was immersed for 12 days in a solution of lead 
acetate (AT), nitric acid {1.6 N). 

The results of the immersion of the lead in the lead-acetate solu- 
tions are summarized in Table 3 . 



228 



Scientific Papers of the Bureau of Standards 



[Vol. i6 



TABLE 3.— Effect of Nitric-Acid Solution of Lead Acetate Upon Lead After 24 Days' 

Immersion 







Solu- 
tion: 
Con- 
cen- 












No. 


Type of 
leado 


tration 
of nitric 
acid in 
TV solu- 
tion of 
lead 
acetate 


Evolution of 
gas 


Color of solu- 
tion 


Amount of 
"sludge" 


Character of 
surface '' 


Behavior upon 
bending ^ 


1 


Ptire.. 


2.0 7V 


Fine bubbles 


Deep lemon 


Considerable 


Very rough 


Broke easily 








appear with- 


yellow. 


deposit; 




upon bend- 








in a few 




about 0.2 of 




ing. 








minutes. 




the speci- 
men had 
d is int e - 
grated. 






2 


...do... 


1.6 AT 


...do 


Lemon yellow. 


Slight deposit 
(0.02 g in 5 
days). 


Rough; a brit- 
tle layer 
formed on 
each side. 


Deep, continu- 
ous cracks 
formed. 


3 


...do... 


.SAT 


A few very fine 


, Light lemon 


Only a few 


Surface was 


Short, wide, 








bubbles af- 


yellow. 


isolated 


etched 


intercrys- 








ter several 




specks. 


enough to 


talline fis- 








hours' im- 






show crys- 


s u r e s 








mersion. 






t a 1 1 i n e 
structure; 
the comers 
and edges 
were much 
roughened. 


opened. 


4 


...do... 


.5 A'' 


Very small 
isolated 


A slight tinge 
of yellow 


None 


Slight"bloom" 
on surface; 


Fine inter- 










crystalline 








bubbles 


could just be 




faint etch 


cracks were 








seen occa- 


detected. 




markings 


opened. 








sionally. 






visible. 




5 


...do... 


(d) 


No bubbles 


Colorless 


do 


Slight"bloom" 
on surface; 


Fine inter- 








detected. 






crystalline 














no evidence 


fissures 














of etching. 


opened. 


6 


Com- 


1.6 A^ 


Bubbles oc- 


Deep lemon 
yellow. 


Most of the 


Only a few 
fragments of 






mer- 




curred with- 


specimen 




- 


cial. 




in less than 
1 minute 
after immer- 
sion. 




had disinte- 
grated into 
"sludge." 


the speci- 
men were 
left; most 
of it was in 
the form of 
"sludge." 





o See Table 2 for composition. *> See Fig. 6. 



= See Fig. : 



i None. 



2. ELECTROLYSIS OF LEAD IN CONCENTRATED NITRIC ACID 

A number of attempts were made to produce the spongy lead, 
noted by Creighton, by means of electrolysis in concentrated 
nitric acid (sp. gr., 1.42). The lead used v^ras the high-grade ma- 
terial listed in Table 2. This was made the cathode of the electro- 
lytic cell, platinum foil being the anode. The lead electrode was 



Rawdon] BHttleness of Lead 229 

approxiraately 4 by i cm by 3 mm; a current of 2 amperes was 
used in most cases, although in some this was increased to 3 am- 
peres. In most cases a volume of electrolyte of 50 cm^ was used. 

A copious evolution of gas occurred at the platinum anode 
when the circuit was closed ; this was a colorless gas which ttuned 
brown when air was admitted into a tube of it. Evidently it was 
nitric oxide. A fine stream of minute gas bubbles was also to be 
seen rising from the lead cathode. This gas apparently dissolves 
in the concentrated acid. Attempts were made to collect it in a 
test tube filled with concentrated nitric acid inverted over the 
cathode. No gas was collected, however, until the action had 
continued for more than 48 hours and the color of the acid within 
the inverted tube had changed to a dark green. The gas, as col- 
lected, was dark brown in color and evidently was nitrogen per- 
oxide. A white, crystalline substance formed on the lead ca- 
thode — lead nitrate. This substance is rather insoluble in the 
concentrated nitric acid, and collected as a heavy deposit on the 
cathode and on the bottom of the flask under this electrode. 

When the action was allowed to continue for some time (for 
examiple, 24 hours) , a black, gritty deposit formed upon the plati- 
num electrode. This deposit usually formed after the solution 
had become rather warm and had evaporated considerably. This 
was the only substance formed which might be mistaken for an 
allotropic form of lead. In one case, when the solution evaporated 
to a very small volume, some of this black deposit was found on 
the lead cathode as well as on the platinum anode. However, all 
attempts to reproduce this condition failed. 

Chemical analysis showed that the black anode deposit was an 
oxide of lead. An oxygen content of 12.95 per cent was found, 
pure lead peroxide, Pb O,, contains 13.35 per cent oxygen. It is 
evident that the electrolysis in concentrated nitric acid is com- 
plicated by several secondary reactions. The production of nitro- 
gen peroxide at the lead cathode and the lead peroxide at the anode 
are instances. Examination of the lead cathodes after the action 
was completed failed to show any pronounced embrittlement of 
the lead. Examination at intervals during the progress of the 
action showed that the attack on the metal often was intercrystal- 
line in its nature. This is illustrated in Fig. 11, which shows the 
surface of the lead cathode at two different stages. The more 
rapid attack along the crystal boundaries is very evident. That 
the lead has not been rendered brittle by the electrolytic action is 
evident upon sharply bending the sheet (Fig. 11 d). 



230 



Scientific Papers of the Bureau of Standards 



[Vol. j6 





(a) 



(b) 





(c) 
Fig. II. — Pure lead used as cathode in electrolysis of concentrated nitric acid 

(a) Surface view of lead cathode after 12 hours' treatment; portions of specimen showed a distinct 

intercrystalline attack. X 8 
(6) Same material as (a) . The intercrystalline nature of the attack is very evident here. X 5a 
(c) Same material after 36 hours' treatment. The nature of the surface suggests that crystals are 

dissolved bodily after intercrystalline attack has started. X 4 
((/) Same material as (o), bent at an angle of 180°. Metal has not been embrittled by electrolytic 

action to any appreciable extent. X 4 



Rawdon] BHttleness of Lead 231 

V. EXPLANATION OF RESULTS 

Most of the impurities which occur in lead are insoluble in the 
metal. This is true particularly of copper, zinc, iron, nickel, 
aluminum, and cobalt, which are only slightly miscible even when 
both metals are in the liquid state. Such imptirities, after solidi- 
fication, will be lodged between the grains of the lead. Tin, 
antimony, and silver are completely miscible in lead in the molten 
state, but almost entirely insoluble in the solid state, with the 
exception of tin. Bach of these three elements forms an eutectic 
series with lead. These, too, will occur between the grains of the 
metallic lead. 

The difference between the solubility of these impurities and 
that of the pure lead comprising the interior of the crystal accounts 
largely for the disintegration of the metal by intercrystalHne 
embrittlement when immersed in a weak acid solution. The 
greater rate of disintegration of the commercial lead as compared 
with the pure lead is due to the larger amounts of these inter- 
crystalline impurities. The greater solubility of the intercrys- 
talHne film of the lead-tin eutectic which must exist in the metal 
of corroded cable sheath (Fig. 2) as compared with that of the 
lead itself accounts for the rapid disintegration of this material 
when immersed in a solution consisting of substances leached out 
of the surrounding concrete. 

It is to be concluded from the results of the experiments made 
that the so-called allotropic or gray lead described by Heller 
represented only a granular condition of the ordinary form of lead, 
the granulation having been brought about by the action oi the 
electrolyte used, primarily the nitric acid, upon the intercrystalline 
impurities. No evidence of allotropy could be obtained in the 
experiments on very pure lead carried out in the manner described 
by Heller, although unmistakable evidence of intercrystalline 
brittleness was secured. The attack of the intercrystalline metal 
in the high-grade lead by solutions of neutral lead acetate is in 
all probability to be partly ascribed to the difference in the 
electrolytic solution potential of the " amorphous intercrystalline 
cement" as compared with the metal of the interior. To this is 
to be added the effect of the slight intercrystalline impiuities. 
The precipitation of lead from the solution in the form of "lead 
trees" in such experiments may be taken as one line of evidence. 
The change in density of lead specimens immersed in lead-acetate 
solutions, noted by Cohen and Helderman, may be ascribed to an 
accompanying oxidation along the grain boundaries, such as 



232 Scientific Papers of the Bureau of Standards [Voi.i6] 

readily occurs in water or weak aqueous solutions on freshly- 
exposed sirrfaces of lead. However, this explanation does not 
completely account for all the changes in density noted. No 
evidence of embrittlement by means of electrolysis could be 
obtained, nor was any product formed, other than a deposit of 
lead peroxide upon the anode, which might be mistaken for an 
allotropic form of lead. 

It is to be concluded that the forms previously described as 
allotropic lead were only a granular condition of the ordinary- 
form brought about by intercrystalline embrittlement, accom- 
panied, perhaps, by slight oxidation. 

VI. SUMMARY 

1 . A type of deterioration of lead which renders the metal weak, 
brittle, and capable of being crumbled easily into grains is 
described. The deterioration occurs as a result of corrosion during 
service; the attack on the metal is localized along the crystal 
boundaries, and the brittleness produced is truly intercrystalline 
in its nature. 

2. Practically all of the commonly occurring impiunties in lead 
are insoluble in the solid state and are to be found lodged between 
the grains of the lead. The intercrystalline brittleness is due 
largely to the behavior of these impurities when the metal is 
immersed in an electrolyte. 

3. Specimens of very pure lead were treated in the manner 
described by previous investigators for the production of the 
allotropic form of lead. No evidence was obtained to justify the 
claim that lead may exist in an allotropic state analogous to the 
well-known gray tin. 

4. The forms of lead previously described in the scientific 
literature as allotropic states appear to be due to an intercrys- 
talline attack by the electrolyte, immersion in which was necessar}^ 
to bring about the allotropic change. The rate at which the 
so-called allotropic transformation occurs is largely a function of 
the purity of the lead and the acidity of the electrolyte in which 
the metal is immersed. 

The author wishes to acknowledge the very efficient help of 
J. F. T. Berliner in the many examinations necessary in the course 
of the investigation. 

Washington, November 13, 1919. 



